This invention relates generally to block and tackle systems and more particularly to tackle systems which can be operated at more than one mechanical advantage by hauling and release of a single line.
Pulleys or bearing blocks are used extensively on sailing craft to allow adjustments to various parts of the rigging and especially the sails. For example, a mainsheet system may be employed between the boom attached to the foot of a mainsail and the aft deck of a boat. These systems typically comprise one or more blocks attached to the boom and one or more blocks attached to the deck, with a single line passing or reeved between successive blocks to provide a simple purchase system, with the mechanical advantage depending on the number of blocks used and the number of falls or passes extending between the blocks. The bitter end of the sheet or line may be either secured to the deck or the boom. The free end of the line is either pulled in or released to control the horizontal and vertical position of the boom. Some of the mainsheet systems in current use are shown on pages 208 and 209 of the 1994 catalog of Harken Yacht Fittings.
Sheeting systems for sails which are subjected to high forces involve various tradeoffs if the sheeting is done by hand. For example, it is important to be able to sheet the sail in and out quickly as the boat is being turned, in order to retain maximum speed, and also for reasons of safety. Such objective dictates a low purchase system.
In contrast, as the mainsheet is repeatedly trimmed and released to fine tune the sail for maximum speeds, the end of the boom is pulled by maximum forces on the sail, and the load on the system is very high. This consideration dictates the need for a block and tackle system having a high purchase. For example, high performance catamarans typically use 8:1 mainsheet systems, and a three foot adjustment of the boom requires the sailor to haul in twenty-four feet of line. If a lower ratio system was employed, the sailor would not be able to easily adjust the sail on a continuous basis without undue effort and fatigue.
When sailing, and particularly under competitive racing conditions, the sheets for the sails are initially trimmed in for a given point of sail. The sheets are then continually eased out and trimmed in, in order to obtain maximum speed, and to accommodate changes in the wind and water. The continued adjustments quickly lead to fatigue if a low purchase system is employed.
For this reason, as indicated above, two separate systems having different ratios have been employed, but these systems are complex and require the use of two separate systems and sheets. Another system is shown in EP 79593, which uses a single line that is doubled back through the system, with the free ends of the line passing through side-by-side cleats. Both lines are moved together for coarse adjustments, and for fine adjustments, one line is secured and the other is moved. In effect, the system uses two sheets, and two free ends must be employed.